1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunctional machine having functions of at least two of the above machines, and a fixing device installed in the image forming apparatus.
2. Discussion of the Background Art
In background image forming apparatuses such as a copier and a printer, an on-demand-type fixing device having a relatively short rise time is widely used.
The on-demand-type fixing device is constituted by an endless fixing film serving as a fixing film, a pressure roller serving as a pressure member, a heater such as a ceramic heater serving as a heating device, and so forth. The heater, which is provided on the inner side of the fixing film and comes into contact with the pressure roller via the fixing film to form a nip portion thereat, heats the fixing film. At the nip portion, a toner image formed on a recording medium conveyed to the nip portion is subjected to heat and pressure and fixed on the recording medium.
The on-demand system involves issues of friction generated between the fixing film and the member in contact with the fixing film (i.e., the heater serving as the heating device in the above example), and improvement of the heating efficiency in heating the fixing film by the heater. There are background techniques addressing the issues.
In a fixing device according to one of the background techniques, the coefficient of friction between a drive roller and a fixing film is set to be greater than the coefficient of friction between another roller and the fixing film. Accordingly, the fixing film can slidingly move smoothly in accordance with the rotation of the drive roller.
In a fixing device according to another one of the background techniques, at least one of the most upstream end portion and the most downstream end portion of a heating member in the moving direction of a fixing film is bent. Accordingly, it is possible to prevent an edge portion of the heating member from coming into contact with the fixing film and deforming a substantially completely circular shape of the movement locus of the fixing film, and to prevent an increase in rotational resistance of the fixing film.
In a fixing device according to another one of the background techniques, a surface of a heating member that comes into contact with and slidingly moves on a fixing film is coated with or includes a fluororesin. Accordingly, the contact resistance of the fixing film is reduced.
In the background on-demand-type fixing devices described above, however, replacement servicing, i.e., maintenance of the heating device such as the heater is a problem, as described in detail below.
Specifically, the life of the heater is limited. In the fixing device, therefore, maintenance to replace the heater with a new one is often performed. However, the heater is in pressure contact with the pressure roller via the fixing film, and thus it is difficult to pull out the pressure-applied heater in the width direction, i.e., the longitudinal direction thereof.
It is conceivable to provide a mechanism for releasing the pressure acting between the heater, the fixing film, and the pressure roller. That is, the heater can be removed from the fixing device after the pressure acting between the heater and the pressure roller is released by the operation of the pressure release mechanism. In this case, however, extra cost and space are required to provide the pressure release mechanism.
In particular, according to the configuration of the background on-demand type fixing devices, the heater is constantly pressed by the pressure member, and thus is likely to be damaged in an unjamming process, during shipment of the devices, and so forth. Therefore, the above issue of the constantly applied pressure is not negligible.
Further, in the background on-demand-type fixing devices described above, the heater is in contact with the fixing film. Therefore, if the fixing devices are provided with a member for reducing the coefficient of friction between the heater and the fixing film or for improving the thermal conductivity, some measure is required that prevents the characteristics of the heater such as the heating efficiency and the safety of the heater from being directly affected by the addition of such a member.